One snag with cloud applications is that they often run over that pesky thing called the public Internet, a Wild West of uncertain speed and reliability. That's fine for a lot of consumers but not always great for businesses.

For companies using Office 365, Microsoft has now rolled out a premium service called Azure ExpressRoute, which allows them connect to Office 365 services via a private, managed wide area network.

ExpressRoute was already available for Microsoft's Azure cloud services, and as of Tuesday it's generally available for Office 365.

The service is currently offered by British Telecom, Equinix, Tata Communications, TeleCity Group and Verizon. Pricing information is here and details of how to sign up are in the blog post.

Businesses that need the extra control will be glad of the option, but it's also an example of the extra costs companies need to consider when they're embarking on a cloud migration.

It's also unlikely that ExpressRoute will avoid all outages. If Microsoft experiences problems with its own data centers, which appeared to be the case during a nine-hour Exchange outage last year, no amount of managed network capacity will help.

Still, the same can be said of on-premises applications, and ExpressRoute offers a better option than the public Internet. Each circuit has two physical connections and two physical routers to provide redundancy, and Microsoft says it provides an uptime SLA (service level agreement) of 99.9 percent on all the Microsoft components.

It doesn't provide an SLA for the network provider's equipment, of course, so customers need to negotiate that separately.

The service is available for Exchange Online, SharePoint Online, OneDrive for Business, Skype for Business Online, Azure Active Directory, Office 365 Video, Power BI, Delve and Project Online.

It doesn't cover all services, and businesses still need to use the public Internet for ProPlus installation files, Yammer, Domain Name Service and Content Delivery Network servers.